Metal gate electrodes are currently being investigated to replace polysilicon gate electrodes in today's ever shrinking and changing transistor devices. One of the principal reasons the industry is investigating replacing the polysilicon gate electrodes with metal gate electrodes is in order to solve problems of poly-depletion. Traditionally, a polysilicon gate electrode with an overlying silicide was used for the gate electrodes in CMOS devices. However, as device feature size continues to shrink, poly depletion becomes a serious issue when using polysilicon gate electrodes.
Accordingly, metal gates have been proposed. However, in order to optimize the performance of CMOS devices, the metal gates need dual tunable work functions. For instance, the metal gates need tunable work functions for NMOS and PMOS devices similar to present polysilicon gate technology, requiring the work functions of metal gates to range from 4.1˜4.4 eV for NMOS and 4.8˜5.1 eV for PMOS (see, B. Cheng, B. Maiti, S. Samayedam, J. Grant, B. Taylor, P. Tobin, J. Mogab, IEEE Intl. SOI Conf. Proc., pp. 91-92, 2001).
Recently, silicided metal gates have been investigated based on the extension of existing self-aligned silicide (SALICIDE) technology. In this approach, polysilicon is deposited over the gate dielectric. A metal is deposited over the polysilicon and reacted to completely consume the polysilicon resulting in a fully silicided metal gate, rather than a deposited metal gate. The silicided metal gate provides a metal gate with the least perturbation to the conventional process, and avoids contamination issues. Furthermore, poly doping has been shown to affect the work function of the silicided metal gates.
The silicided metal gates are not without their problems. One of the more recognizable problems associated with the silicided metal gates may be attributed to the difficulty in evenly and consistently fully siliciding the silicided metal gates. It has been observed in various prior art devices that the silicided metal gates do not have a consistent silicidation across the thickness of the metal gate across the wafer. This, as one would expect, at a very minimum would lead to inconsistent electrical properties for the silicided metal gates, and in extreme situations, device failure.
Accordingly, what is needed is a method for manufacturing silicided metal gate structures that does not experience the drawbacks of the prior art methods.